Such treatment operations are generally carried out over metal catalysts deposited on an amorphous or crystalline carrier. The metals used are metals of group VIII, for example, nickel and palladium.
The highly unstable character of such pyrolysis volatiles makes treatment thereof relatively difficult because, simultaneously with the hydrogenation effect, a polymerisation reaction occurs on the catalyst, which causes clogging and deactivation of the catalyst. In order to compensate for that loss of activity, the operating temperature is progressively increased but that mode of procedure further increases the rate at which polymeric deposits occur. In consequence it is necessary periodically to halt operation in order to carry out a combustion operation on the catalyst in order to restore its initial activity. Halting the operation represents a real loss of production and the combustion operation has to be carried out with a very high degree of precision in order to avoid irreversible degradation of the properties of the catalyst. Any improvement in the process which will permit an increase in the cycle time, that is to say the period of time between two combustion operations, will substantially enhance the quality of the process.
Carrying out the hydrogenation operation itself involves a system for the removal of heat, as the degree of exothermicity is such that the catalyst would be damaged by the excessively high temperatures which occur at the discharge from the catalyst bed. The above-indicated operation of removing heat can be effected by exchange with a heat exchange fluid in a reactor-exchanger, the catalyst being kept in the tubes and the heat exchange fluid being discharged at the shell side. Such a procedure, which is referred to as isothermal, is complicated and requires the use of highly burdensome reactors.
The use of chamber-type reactors is generally preferred and control of the exothermicity of the reaction is effected by substantial recycling of hydrogenated product to the top of the bed. One improvement involves dividing the catalyst into two beds and cooling the effluent from the first bed by means of a quench liquid formed by cold hydrogenated product.
Nonetheless, such a procedure is not entirely satisfactory as the whole of the catalyst is subjected to the polymerization effect, which in many cases causes a premature stoppage of the operation due to an excessive pressure drop at the intake to the section.